avfilter/asrc_flite: Fix races upon (un)registering voices

The voice registration system in libflite is broken: It is not
thread-safe and also not based on internal counters; instead
any call to unregister a voice frees said voice even if there are still
many other users of said voice who have also registered said voice.
While there is no way to guard against another library unregistering
voices behind our back, we can at least be correct in the absence of
other users of libflite. The current code already tried this by using
a reference count of our own for each voice; but the implementation
of this is not thread-safe at all.

Fix this by using a mutex to guard all of libavfilter's libflite
registration and unregistration calls, thereby being thread-safe
in the absence of other libflite users.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

1 files changed, 21 insertions, 8 deletions

diff --git a/libavfilter/asrc_flite.c b/libavfilter/asrc_flite.c
index bd2ae774de..6335a81f0e 100644
--- a/libavfilter/asrc_flite.c
+++ b/libavfilter/asrc_flite.c
@@ -27,6 +27,7 @@
 #include "libavutil/channel_layout.h"
 #include "libavutil/file.h"
 #include "libavutil/opt.h"
+#include "libavutil/thread.h"
 #include "avfilter.h"
 #include "audio.h"
 #include "formats.h"
@@ -63,7 +64,9 @@ static const AVOption flite_options[] = {
 
 AVFILTER_DEFINE_CLASS(flite);
 
-static volatile int flite_inited = 0;
+static AVMutex flite_mutex = AV_MUTEX_INITIALIZER;
+
+static int flite_inited = 0;
 
 /* declare functions for all the supported voices */
 #define DECLARE_REGISTER_VOICE_FN(name) \
@@ -111,14 +114,19 @@ static int select_voice(struct voice_entry **entry_ret, const char *voice_name,
     for (i = 0; i < FF_ARRAY_ELEMS(voice_entries); i++) {
         struct voice_entry *entry = &voice_entries[i];
         if (!strcmp(entry->name, voice_name)) {
+            cst_voice *voice;
+            pthread_mutex_lock(&flite_mutex);
             if (!entry->voice)
                 entry->voice = entry->register_fn(NULL);
-            if (!entry->voice) {
+            voice = entry->voice;
+            if (voice)
+                entry->usage_count++;
+            pthread_mutex_unlock(&flite_mutex);
+            if (!voice) {
                 av_log(log_ctx, AV_LOG_ERROR,
                        "Could not register voice '%s'\n", voice_name);
                 return AVERROR_UNKNOWN;
             }
-            entry->usage_count++;
             *entry_ret = entry;
             return 0;
         }
@@ -141,12 +149,15 @@ static av_cold int init(AVFilterContext *ctx)
         return AVERROR_EXIT;
     }
 
+    pthread_mutex_lock(&flite_mutex);
     if (!flite_inited) {
-        if (flite_init() < 0) {
-            av_log(ctx, AV_LOG_ERROR, "flite initialization failed\n");
-            return AVERROR_UNKNOWN;
-        }
-        flite_inited++;
+        if ((ret = flite_init()) >= 0)
+            flite_inited = 1;
+    }
+    pthread_mutex_unlock(&flite_mutex);
+    if (ret < 0) {
+        av_log(ctx, AV_LOG_ERROR, "flite initialization failed\n");
+        return AVERROR_EXTERNAL;
     }
 
     if ((ret = select_voice(&flite->voice_entry, flite->voice_str, ctx)) < 0)
@@ -197,10 +208,12 @@ static av_cold void uninit(AVFilterContext *ctx)
     FliteContext *flite = ctx->priv;
 
     if (flite->voice_entry) {
+        pthread_mutex_lock(&flite_mutex);
         if (!--flite->voice_entry->usage_count) {
             flite->voice_entry->unregister_fn(flite->voice);
             flite->voice_entry->voice = NULL;
         }
+        pthread_mutex_unlock(&flite_mutex);
     }
     delete_wave(flite->wave);
     flite->wave = NULL;